The copper and its alloy are a kind of nonferrous metals that are used comprehensively for many proposes. It was frequently used as early as thousands years ago. For example, in Yin and Zhou dynasty (more than 3700 years ago), Chinese people are well known for the manufacturing on the bells, tripods (ancient cooking vessel with two loop handles and three or four legs) as well as weapons by bronze. So far, Cu and its alloys are still extensively used in conventional and modern industry. The main characteristics of Cu and its alloys are high electrical conductivity, good thermal conductivity, also good corrosion resistance in atmosphere, seawater and many other mediums. Moreover, they have very good plasticity and wear resistance, which are suitable for processing and casting various kinds of products. The copper and its alloys are the indispensable metal materials in many industrial fields, such as electric power, electrician equipment, thermal technology, chemical industry, instrument, shipbuilding and machine-manufacturing, etc.
The pure Cu has a very good conductive performance. However, the strength is pretty low. Strengthening Cu and its alloys could be approached by several methods, such as grain refinement, cold working, solid solution alloying etc, but such approaches usually lead a pronounced decrease in conductivity. For example, alloying pure Cu by adding elements (Al, Fe, Ni, Sn, Cd, Zn, Ag, Sb etc.) may increase the strength by two or three times, but the electrical conductivity of Cu alloys will decrease dramatically. Otherwise, adding minim Fe and Ni will affect the magnetic property of Cu, which is a disadvantage to making compasses and aviation instrument. The volatilities of some alloy elements, such as Cd, Zn, Sn and Pb etc., would limit their application in electronic industry, especially in high temperature and high vacuum environments. Currently, machine equipment, toolmaking and instrument apparatus are going for a high speed, high efficiency, high sensitivity, low energy consumption and microminiaturization. Therefore the high and comprehensive demand for copper material has been presented in precision and reliability. For instance, the new-type high performance of copper material is urgently required in the rapidly developing computer industry, automobile industry, radio communication (such as plug connector in cell phone and lithium battery) and printing (for making the multi-layer printed circuit board and high density printed circuit board) etc. So there are great challenges to significantly strengthening copper and its alloys without damaging their excellent electrical conductivity.
The nanocrystalline materials refer to single phase or multiphase solid materials consisting of very fine grains of 1-100 nm in diameter. Due to its small grain and numerous grain boundaries (GBs), nanocrystalline materials are expected to exhibit tremendous difference from conventional micron-sized polycrystalline materials in physical and chemical performances, such as mechanics, electrics, magnetics, optics, calorifics, chemistry etc.
Grain refinement is often used to strengthen materials in engineering, which increases the strength of materials by introducing more grain boundaries to obstacle dislocation motion, described by the well-known Hall-Petch (H-P) relationship as σy=σ0+d−1/2. However, the strength does not monotonously increase with decreasing grain sizes in any regiem; when the grain size reduces down to nanometer scale, especially less than a critical size, abnormal H-P relationship will occur. Actually, both experimental observations and computer simulations have shown that the strengthening effect will weaker or disappear as the grain sizes are refined to nanometer, thereby softening effect appears. When grain sizes are small enough, namely close to lattice dislocation equilibrium distance, few dislocations can be accommodated in grains, and grain boundary activities (e.g. grain boundary rotating and sliding) would be dominate, leading to the softening of materials. Therefore, for nanocrystalline materials, ultrahigh strength can be achieved by suppressing the dislocation activities and the grain boundary activities simultaneously.
Strengthening of solid solution alloying or introduction of a second phase is also effective method in blocking the motion of lattice dislocations. Cold-working (plastic straining), which generates numerous dislocations during deformation process and limits the further dislocation activities, also strengthen the materials. All of these strengthening approaches are based on the introduction of various kinds of defects (GBs, dislocations, point defects and reinforcing phases, etc.), which restrict dislocation motion but increase the scattering for the conducting electrons. The latter will decrease the electrical conductivity of materials.
For example, the tensile yield strength (σy) of the coarse-grained Cu at room temperature is only 0.035 GPa, which is about two orders of magnitude lower than the theoretical strength, and the elongation is about 60%. After cold-working (as-rolled Cu), the tensile yield strength increases appropriately, being about 250 MPa. Nanocrystalline Cu has higher σy than coarse-grained Cu. American scientists J. R. Weertman et al. [Sander P. G, Eastman J. A. & Weertman J. R., Elastic and tensile behavior of nanocrystalline copper and palladium, Acta Mater., 45 (1997) 4019-4025] produced nanocrystalline Cu by inert-gas condensation with grain sizes of about 30 nm, and the tensile yield strength is 365 MPa at room temperature. Prof. R. Suryanarayana et al. [Suryanarayana R. et al., Mechanical properties of nanocrystalline copper produced by solution-phase synthesis, J. Mater. Res. 11 (1996) 439-448] prepared nanocrystalline copper powder by ball milling, then cold-pressed the purified Cu powder to nanocrystalline Cu with the grain size of 26 nm, it's yield strength is about 400 MPa. However, nanocrystalline samples have very limit elongations, usually less than 1-2%. In China, L. Lu, K. Lu et al. (patent application numbered 0114026.7) produced bulk nanocrystalline Cu with the grain sizes of 30 nm by electrodeposition technique. It is indicated that the as-deposited nanocrystalline Cu consisted of small-angle GBs, unlike the large-angle GBs in conventional nanometer materials. The yield strength at room temperature is 119 MPa and the elongation 30%.
If the as-deposited nanocrystalline Cu was cold-rolled at room temperature, the average grain sizes of the sample remained unchanged, but the misorientation among the nanocrystallites and the dislocation density increased. The yield strength of the as-rolled nanocrystalline Cu reached as high as 425 MPa, but the elongation declined to 1.4%. J. R. Weertman et al. achieved the yield strength of 535 MPa in microsample tensile testing of nanocrystalline Cu specimen (1 mm) [Legros M., Elliott B. R., Ritter M. N., Weertman J. R. & Hemker K. J., Microsample tensile testing of nanocrystalline metals, Philos. Mag. A., 80 (2000) 1017-1026]. For the nanocrystalline Cu samples produced by surface mechanical attrition treatment, the tensile results at room temperature of the microsamples (thickness of the sample 11-14 μm, gauge length 1.7 mm, cross-section area 0.5 mm×0.015 mm) showed that the yield strength was as high as 760 MPa, but the elongation was almost zero [Wang Y. M., K. Wang, Pan D., Lu K., Hemker K. J. and Ma E., Microsample tensile testing of nanocrystalline Cu, Scripta Mater., 48 (2003) 1581-1586]. Meanwhile, the yield strength of about 400 MPa is achieved in compression testing at room temperature for the copper with the grain size of 109 nm processed by severe plastic deformation, however, the electrical resistivity at room temperature (293 K) was as high as 2.46×10−8 Ω·m (only 68% IACS) [Islamgaliev R. K., Pekala K., Pekala M. and Valiev R. Z., Phys. Stat. Sol. (a), 162 (1997) 559-566].